Kramer being stuck in the apartment for 10 years

I've never been too clear on this and tried looking at all of the old episodes, but in the pilot it's discussed how Kramer hasn't been out of the apartment building in like 7 or 10 years (can't remember exactly). Then I don't think he's featured for a couple more episodes. What's the deal here? Did they ever follow up with this, or just drop it at that?